Electronics for Telecommunications
Research Laboratory

Low-Frequency Dynamic I/V Measurements

As an alternative to pulsed measurement systems, a low-frequency setup has been developed to measure dynamic I/V transistor characteristics under sinusoidal excitations. This system is based on the acquisition of incident and reflected waves using directional couplers and sampling oscilloscopes. It employs two signal sources (or a single source and a passive tuner) to implement a harmonic-controlled low-frequency load-pull (LFLP) setup, enabling in-depth characterization of dynamic I/V characteristics under various realistic operating conditions.

The LFLP setup has been successfully applied in:

• transistor characterisation

• transistor modeling

• power amplifier design

• reliability evaluation.

Dynamic-Bias Nonlinear Measurements

Traditionally, microwave transistor models are described using nonlinear conductive currents and charges, combined with parasitic elements accounting for the device layout. Transistor currents and charges can be directly obtained from measurements and stored in look-up tables or represented through analytical expressions. While DC and small-signal measurements already provide valuable information, nonlinear measurements have become increasingly important in recent decades, facilitated by instruments like Large-Signal Network Analyzers and Nonlinear Vector Network Analyzers. These tools, currently operating up to 67 GHz, enable vector-calibrated nonlinear measurements and the acquisition of time-domain waveforms at transistor terminals, which can be used to extract current and charge values.

We proposed an alternative approach that combines the capabilities of nonlinear measurement systems operating at different frequency ranges. The proposed technique is based on the concept of "dynamic-bias," where the main nonlinearities of the device under test (DUT) are induced by large low-frequency signals (in the megahertz range) combined with a small RF signal. More precisely, a low-frequency load line is imposed to the DUT by using the LFLP setup described above and a high-frequency tikle is super-imposed along such a load line. Unlike traditional methods, this approach does not require the acquisition of RF harmonics, allowing the RF frequency to be set as high as the instrument permits. Additionally, this method significantly reduces measurement time by capturing the main transistor nonlinearities using a limited set of experimental data. 

This technique has been successfully adopted to identify non-quasi-static millimeter-wave large-signal FET models. Moreover, dynamic-bias S-parameters can be defined and measured, offering an interesting and more powerful alternative to pulsed S-parameter measurements.